The neurologic origins of both normal and pathological social behavior remain poorly understood. The study of naturally occurring sex differences in social behavior provides an opportunity for discovery of unique variables that would never be revealed by the study of one sex in isolation. The amygdala is a brain region critically involved in emotional and social behaviors and characterized by a complexly regulated sexually dimorphic profile that varies across the lifespan. Little attention has been focused on the cellular mechanisms by which sex differences in the amygdala arise and the consequences this might have for social behavior. We recently made the novel discovery of a sex difference in cell genesis in the developing amygdala of the neonatal rat, observing more new cells being generated in female pups compared to their male littermates. We further observed that while most of the new cells being born postnatally in both sexes will become neurons, the higher rate of cell genesis in females is the result of more astrocytes being generated. Astrocytes are critical partners in synaptogenesis and neural functioning. We also made the surprising discovery that males have a higher overall endocannabinoid tone in the developing amygdala and that pharmacologically increasing the tone in females reduces the rate of cell genesis to that of males. Endocannabinoids are lipid derived signaling molecules that ubiquitously regulate synaptic function and are only recently being recognized for their role in cell proliferation. Lasty, the same treatment that increases endocannabinoid tone and cell genesis in newborn females masculinizes their social play behavior as juveniles. We now propose three specific aims to 1) determine whether gonadal steroid hormones mediate the sex difference in endocannabinoid tone, 2) identify the signal transduction pathway(s) mediating the sex difference in cell genesis and its regulation by endocannabinoids and 3) determine the impact of the higher rate of astrocyte proliferation in females on amygdala neuron synaptic profile and its correlation with control of juvenile social play. Standard techniques will be used to quantify cell genesis (BrdU incorporation), neuron and astrocyte morphology (Golgi/Cox, immunocytochemistry, confocal microscopy), endocannabinoids and associated enzymes (GS/MS, Western blot) and to pharmacologically manipulate the endocannabinoid system and associated kinase pathways and measure social behavior. The data generated by these experiments will greatly expand our understanding of the neural control of social behavior and identify novel nodes of importance which may provide new targets for prevention and therapeutic intervention.